A typical backlight module generally includes a backplane, a middle frame, a light source, a light guide device, and an optical film. The backplane is generally formed by stamp molding of a sheet metal member. As shown in FIG. 1, a protrusion structure 8 is a feature on the backplane 1. A stretched height of the protrusion structure 8 has a close relationship with size of the protrusion structure 8. Generally, the stretched height of the protrusion structure 8 is less than or equal to half of a diameter of the protrusion structure 8. If higher stretching of protrusion structure is adopted, the protrusion structure 8 may tear at a base of the protrusion structure 8, and strength of backplane is affected.
FIG. 2 shows a general flow process of stretching the protrusion structure on a thin wall sheet metal member. Volume of the stretching part of the sheet metal member is not changed by the molding of the protrusion structure, but an area of the sheet metal member is expanded by stretching and a wall thickness of the sheet metal member is thinned, or the wall thickness of the protrusion structure is thinned compared with other parts on the sheet metal member.